Current thermophysical measurement techniques experience difficulty in acquiring transport properties along the length of very small specimen (such as, by way of example, carbon nanotube “yarns”). Such materials have lateral dimensions of tens or hundreds of microns and exhibits substantially different thermal conduction in their axial and transverse orientations due its inherent directional lattice morphology. Modeling the bulk properties of composites containing micro-scale or nano-scale fibers or particulates requires knowledge of the properties of the individual constituents, because the nanoscale/microscale thermophysical properties of materials are markedly different from their macroscale properties.
Many other materials, such as nonmetallic fibers or nanotube forests grown on 2 dimensional surface, exhibit strong anisotropic thermal behavior due to structural characteristics. Some materials possess multiphase mixtures and will have very different thermal properties at the microscopic dimension of the phases and at the boundaries between these phases. Still other cases exist where the crystal structure itself is highly anisotropic, leading to quite disparate thermal transport such as that found in layered materials like graphite or perovskite ceramics. In fact, many new materials are designed and fabricated at the microscopic level and below, and do not have macroscopic bodies that can be tested with conventional thermal measurement apparatus.
In each of the cases noted above, traditional methods of measuring thermal conduction through a bulk material cannot be made, and they do not accurately reflect the thermophysical properties of the microscopic or nanoscopic constituents of a material or structure.
Though there is an abundance of theoretical studies on thermal transport properties of micro- and nano-structured materials, experimental measurement at these length scales are still challenging and thus scarce. Conventional thermal measurement techniques with devices at the micro- and nano-meter length scales do not have sufficient spatial resolution to determine accurate temperature and resulting thermal conductivity. Recently, several microscale temperature measurement techniques have been investigated by different groups using 1) far-field optical techniques, such as infrared emission, laser surface reflectance, and liquid crystal microscopy, 2) near-field optical thermometry techniques, and 3) non-optical techniques, such as scanning thermal microscopy (SThM) and a photo acoustic technique.
The spatial resolution of the far-field optical techniques is limited by optical diffraction wavelength to the order of approximately 1 μm or larger. With the near-field optical techniques, the spot size could be reduced to a fraction of the wavelength by using a solid immersion lens. Near-field thermometry has reportedly enabled sub-micrometer spatial resolution, and SThM is capable of thermally resolving sub-100 nm features with a proper design of SThM probes. While the noted conductivity measurement methods have their own merits, these indirect measurement techniques tend to yield qualitative rather than quantitative data and often rely on modeled localized thermal flow.
Another measurement technique applied to the microscale specimens is thermal conductance evaluation. Among different thermal conductance schemes, a 3ω method is the preferred choice for the in-plane thermal conductivity measurement of thin films. However, the 3ω method does not produce acceptable results for the thermal property measurements of freestanding microscale structures such as fibers.
One possible apparatus for measuring freestanding microscale structures includes a suspended micro-device for measuring the thermal conductivity of silicon nanowires of different diameters (ranging from 22 nm to 115 nm) over a temperature range from 20K to 320K. The apparatus includes two silicon nitride (SiN) membranes suspended by five SiN beams. A thin Pt resistance coil and a separate Pt electrode are patterned onto each membrane. Each Pt resistor serves as a heater to raise the temperature of the suspended elements and also as a resistance thermometer to measure the temperature of each element. Si nanowires are then drop-cast using propanol dispersion to bridge the gap between the two SiN membranes. The entire test procedure may be performed under vacuum inside a Scanning Electron Microscope (SEM), and thermal conductance may be ascertained. Unfortunately, prior systems are only amenable to randomly oriented nanofibers that can be transported to the fixed gap between these heaters and there is no independent control of test temperature and heat flux. Therefore, the test temperature and heat flux through the specimen are dependent on the resistive heating of the Pt elements. Due to this, to maintain a fixed temperature during testing active cooling may be required in prior art configurations.
Therefore, there exists a need for improved methods and apparatus for performing thermal conductance measurements on a wide variety of ultra small specimens.